The present invention relates to the aqueous emulsion polymerization of fluorinated monomers to produce specific fluoropolymers, in particular to produce fluorothermoplasts or fluoroelastomers. Specifically, the present invention relates to an improvement in the aqueous emulsion polymerization of fluorinated monomers wherein no emulsifier is added.
Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc . . . The various fluoropolymers are for example described in xe2x80x9cModern Fluoropolymersxe2x80x9d, edited by John Scheirs, Wiley Science 1997.
The known fluoropolymers include in particular fluoroelastomers and fluorothermoplasts. Such fluoropolymers are generally copolymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as for example hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P).
Examples of fluoroelastomers include for example copolymers of TFE and PVE and copolymers of VDF and HFP. The fluoroelastomers may also contain cure site components so that they may be cured if desired. Applications of fluoroelastomers include for example coatings, use as gaskets and seals as well as use as polymer processing aids (PPA). A commercially available processing aid includes for example copolymers of VDF and HFP available from Dyneon LLC under the brand DYNAMAR(trademark) PPA.
Examples of fluorothermoplasts include semicrystalline copolymers of TFE and E (ETFE), copolymers of TFE and HFP (FEP), copolymers of TFE, HFP and VDF (THV) and perfluoroalkoxy copolymers (PFA). Examples of applications of fluorothermoplasts include for example coating applications such as for example for coating outdoor fabric and use as insulating material in wire and cable insulation. In particular ETFE copolymers have desirable properties as insulating material. Further applications of fluorothermoplasts include making of tubes such as for example fuel hoses, extrusion of films and injection molded articles. The extruded fluorothermoplastic articles, in particular films may further be subjected to an e-beam radiation to partially cure the fluorothermoplast.
Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. Nos. 3,855,191, 4,439,385 and EP 649863; aqueous emulsion polymerization as disclosed in e.g. U.S. Pat. Nos. 3,635,926 and 4,262,101; solution polymerization as disclosed in U.S. Pat. Nos. 3,642,742, 4,588,796 and 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used for the stabilization of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally good stability over a long period of time is obtained.
An aqueous emulsion polymerization wherein no surfactant is used has been described in U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381 to generally produce homo- and copolymers of chlorotrifluoroethylene (CTFE). For example, WO 97/17381 discloses an aqueous emulsion polymerization in the absence of a surfactant wherein a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization and whereby the initiator system is added in one or more further charges during the polymerization. However, the aqueous emulsion polymerization process disclosed there has the disadvantage that a dual feed of reducing agent and oxidizing agent is required, making the process more cumbersome. This means in practice, for example, that additional feeding lines and control devices are needed and the dual feed inevitably increases the risk of failures during the polymerization. Also, WO 97/17381 mainly relates to CTFE polymers and does not disclose improved properties that may be obtained for fluoropolymers other than CTFE polymers.
The aqueous emulsion polymerization process in the presence of fluorinated surfactants is a desirable process to produce fluoropolymers because it can yield stable fluoropolymer particle dispersions in high yield and in a more environmental friendly way than for example polymerizations conducted in an organic solvent. However, for certain applications, the fluoropolymers produced via the aqueous emulsion polymerization process may have undesirable properties relative to similar polymers produced via solution polymerization. For example, purity is required for polymers used in applications with food contact, and in particular the presence of extractables (e.g., fluorinated surfactants and other low molecular weight compounds) is highly regulated. Furthermore, fluorinated surfactants typically used in aqueous emulsion polymerization such as perfluoro octanoic acid or perfluoro sulfonic acids are expensive and are considered as environmental concern nowadays. It is therefore desirable to run aqueous emulsion polymerizations in the absence of surfactants without however compromising the properties of the polymers resulting.
It would also be desirable to improve the aqueous emulsion polymerization process so that also fluoropolymers of higher quality can be produced meeting the needs of demanding applications. In particular, it would be desirable to improve properties such as the mechanical and physical properties of the resulting polymer, the purity level, reducing the amount of extractable substances, reduce discoloration, improved processability and improving performance of the fluoropolymer such as for example the compression set and permeation in case of a curable fluoroelastomer.
The present invention provides a method of making a fluoropolymer comprising repeating units derived from at least one first and at least one second monomer that are different from each other. The fluoropolymers are thus copolymers. The term copolymer in connection with the present invention includes binary copolymers, i.e. copolymers of only two different monomers, as well as copolymers that comprise more than two different monomers such as terpolymers and quaterpolymers. The fluoropolymers may have a partially or fully fluorinated backbone. In one aspect of the invention, the first monomer is a fluoroolefin selected from tetrafluoroethylene (TFE) and vinylidene fluoride (VDF) and the second monomer is at least one comonomer selected from the group consisting of a perfluoroalkyl vinyl monomer such as hexafluoropropylene (HFP), ethylene, propylene, fluorinated allyl ethers and fluorinated vinyl ethers, in particular perfluorovinyl ethers (PVE), vinylfluoride and vinylidene fluoride (VDF). The method comprises an aqueous emulsion polymerization of the first and second monomers in absence of added surfactant (hereinafter also referred to as emulsifier free polymerization) using a redox system as initiator system. In one aspect, the initiator system is a mixture of an oxidizing agent and a reducing agent and this system is used to start the polymerization. During the polymerization there is then further added either one of the oxidizing agent or the reducing agent but not both.
In another aspect of the invention, the emulsifier free polymerization involves an initiator system that comprises one or more fluoroolefin that are capable of reducing an oxidizing metal ion and the oxidizing metal ion. In this system, the initiating species form in situ. Typical oxidizing metal ions include those deriving from potassium permanganate, Mn3+-salts, potassium per-rheanate, Ce4+-salts, etc. These oxidizing metal ions can be used with for example tetrafluoroethylene and/or vinylidene fluoride as the fluoroolefin. The polymerization further involves the uses of a comonomer selected from the group consisting of a perfluoroalkyl vinyl monomer such as hexafluoropropylene (HFP), ethylene, propylene, fluorinated allyl ethers and fluorinated vinyl ethers, in particular perfluorovinyl ethers (PVE), vinylidene fluoride (VDF) and vinylfluoride. In this case, the oxidizing metal ion in combination with the fluoroolefin is used to initiate the polymerization and the metal ion is added further during the polymerization. The fluoroolefin may also be further added during the polymerization as is commonly done in the aqueous emulsion polymerization of fluoroolefins.
By the term xe2x80x9cin absence of added surfactantxe2x80x9d is meant that no surfactant is added to the polymerization system.
The method of the present invention has the advantage that it is more easy and convenient to practice than methods of the prior art while still allowing production of the fluoropolymer in high yield and high polymerization rates. Accordingly, the process of the present invention is easy, convenient and cost effective. Furthermore, the resulting polymer dispersions have good latex stability (that means the latex does not settle or coagulate) despite the fact that the average particle size of the polymers may be as large as 500 nm. Additionally, the fluoropolymers produced with the process of the invention, have a higher purity and less extractable substances and generally yield fluoropolymers that have similar or even improved properties compared to like polymers produced in the presence of added fluorinated surfactant.
Additionally, it has been found that the emulsifier free polymerization method of this invention can be used to produce fluoropolymers that have a multi-modal, e.g., a bimodal, molecular weight distribution in a single step polymerization. By single step polymerization is meant that the polymerization can be carried out without having to interrupt the reaction as has been practiced in the prior art. Such polymerization creating a multi-modal molecular weight distribution, are typically carried out in the presence of chain transfer agents.
The present invention relates to the making of fluoropolymers that comprise repeating units derived from a fluoroolefin selected from the group consisting of TFE and/or VDF and repeating units derived from at least one comonomer selected from the group consisting of E, P, perfluoro alkyl vinyl monomers such as e.g. hexafluoropropylene (HFP), fluorinated allyl ethers and fluorinated vinyl ethers, in particular PVE, vinylidene fluoride and vinylfluoride. It will be clear that in case VDF is selected as the only fluoroolefin, the comonomer should be other than VDF so as to achieve a copolymer.
Examples of suitable PVE monomers include those corresponding to the formula:
CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rfxe2x80x83xe2x80x83(I)
wherein Rf represents a perfluorinated aliphatic group that may contain one or more oxygen atoms. Preferably, the perfluorovinyl ethers correspond to the general formula:
CF2xe2x95x90CFO(RfO)n(Rxe2x80x2fO)mRxe2x80x3fxe2x80x83xe2x80x83(II)
wherein Rf and Rxe2x80x2f are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rxe2x80x3f is a perfluoroalkyl group of 1-6 carbon atoms. Examples of perfluorovinyl ethers according to the above formulas include perfluoro-2-propoxypropylvinyl ether (PPVE-2), perfluoro-3-methoxy-n-propylvinyl ether, perfluoro-2-methoxy-ethylvinyl ether, perfluoromethylvinyl ether (PMVE), perfluoro-n-propylvinyl ether (PPVE-1) and
CF3xe2x80x94(CF2)2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2.
Suitable fluoroalkane monomers correspond to the general formula:
CF2xe2x95x90CFxe2x80x94Rdfxe2x80x83xe2x80x83(III)
or
CH2xe2x95x90CHxe2x80x94Rdfxe2x80x83xe2x80x83(IV)
wherein Rdf represents a perfluoroalkyl group of 1 to 10, preferably 1 to 5 carbon atoms. A typical example is hexafluoropropylene.
The fluoropolymers are produced according to an aqueous emulsion polymerization process in the absence of the addition of an emulsifier. Despite the fact that no emulsifier is added, stable polymer dispersions are produced.
The initiator system used in the aqueous emulsion polymerization process of the present invention is a redox system of an oxidizing agent and a reducing agent. Suitable oxidizing agents include persulfates including for example ammonium persulfate, (APS) potassium persulfate (KPS) and sodium persulfate, preferably APS or KPS. Suitable reducing agents include sulfites, such as sodium sulfite, sodium bisulfite, a metabisulfite such as sodium or potassium bisulfite, pyrosulfites and thiosulfates, preferably Na2S2O5. Other redox systems can be used as well to initiate the polymerization although the aforementioned redox couples are preferred for use with this invention as they generally yield more stable latices.
According to a further embodiment, involving a fluoroolefin such as tetrafluoroethylene and/or vinylidenefluoride, oxidizing metal-ions, such as those deriving from potassium permanganate, Mn3+-salts (like manganese triacetate, manganese oxalate, etc.), potassium per-rheanate, Ce4+-salts, etc. are used to initiate the polymerization. The preferred metal salt is KMnO4. For example, a polymerization of tetrafluoroethylene and further comonomers as disclosed above may be initiated by adding thereto potassium permanganate. During the polymerization potassium permanganate is further added in one or more portions or continuously.
Tetrafluoroethylene and the other comonomers may be further added as well during the polymerization. The benefit of such an initiator system is that only an oxidizing agent (e.g. KMnO4) is added to initiate the polymerization and to continue the polymerization. In certain cases a complexing agent (e.g. oxalic acid, or salts thereof) might be added to avoid precipitation of the active metal complexes, but this is not a necessity.
The aqueous emulsion polymerization process is otherwise generally conducted in the commonly known manner.
Any quantity of the fluoroolefin(s) and comonomer(s) may be charged to the reactor vessel. The monomers may be charged batchwise or in a continuous or semicontinuous manner. By semicontinuous is meant that a plurality of batches of the monomer are charged to the vessel during the course of the polymerization. The independent rate at which the monomers are added to the vessel will depend on the consumption rate of the particular monomer with time. Preferably, the rate of addition of monomer will equal the rate of consumption of monomer, i.e. conversion of monomer into polymer.
The reaction vessel is charged with water, the amounts of which are not critical. Generally, after an initial charge of monomer, the initiator system is added to the aqueous phase to initiate the polymerization. If a mixture of oxidizing agent and reducing agent is used as the initiator system, either of the oxidizing agent or reducing agent may be added first to the aqueous phase followed by the addition of the other agent of the redox system. The initial amount of the initiator system (combined amount of oxidizing and reducing agent) added is typically between 0.01 and 0.2% by weight, preferably between 0.02 and 0.12% by weight based on the total amount of polymer dispersion produced. The molar ratio of reducing agent to oxidizing agent in the initial charge is generally between 1/20 and 1/2, preferably between 1/10 and 1/4. During the polymerization reaction, further amounts of either the reducing agent or oxidizing agent are added. The further addition of reducing agent or oxidizing agent during the polymerization may be carried out as a continuous feed or in separate discrete charges. If for instance the reducing agent is continuously charged into the vessel throughout the polymerization, the feeding rate typically chosen will ensure that an equimolar amount of oxidizing agent to reducing agent is attained after six hours polymerization time. Accelerators such as for example water soluble salts of iron, copper and silver may preferably be added.
In cases where only an oxidizing metal complex, (e.g. KMnO4) is used as part of the initiator system; the amount of initiator continuously added throughout the polymerization is typically between 0.001 and 0.3% by weight, preferably between 0.005 and 0.1% by weight based on the total amount of polymer dispersion produced.
During the initiation of the polymerization reaction, the sealed reactor vessel and its contents are pre-heated to the reaction temperature. Preferred polymerization temperatures are 10 to 100xc2x0 C., preferably 30xc2x0 C. to 80xc2x0 C. and the pressure is typically between 2 and 30 bar, in particular 5 to 20 bar. The reaction temperature may be varied to influence the molecular weight distribution, i.e. to obtain a broad molecular weight distribution or to obtain a bimodal distribution.
The initial temperature to start the polymerization can be set higher, for example 10xc2x0 C. to 50xc2x0 C. higher, than during the rest of the polymerization to ensure a fast initiation rate; the time for this initiation period where the polymerization is carried at a higher temperature can be from 5 min to 60 min from the start of the polymerization reaction. The use of a higher temperature during an initial period may be beneficial for both the redox-system comprising an oxidizing and reducing agent as well as for the initiation system based on an oxidizing metal ion.
The aqueous emulsion polymerization system may further comprise auxiliaries, such as buffers and, if desired, complex-formers or chain-transfer agents. According to a preferred embodiment in connection with the invention, a chain transfer agent is used to adjust the desired molecular weight of the fluoropolymer. Preferably, the chain transfer agent is an alkane or a dialkyl ether, in particular methane, ethane, tertiary butyl methyl ether and/or dimethyl ether. The dialkyl ethers comprise partially fluorinated ethers of the general structure Rfxe2x80x94Oxe2x80x94CH3, whereby Rf can be a linear or branched partially or perfluoro-vest of C1-C10. The dialkyl chain transfer agent concentration may also be varied throughout the polymerisation to influence the molecular weight distribution, i.e. to obtain a broad molecular weight distribution or to obtain a bimodal distribution.
It has been found that the dialkyl ether chain transfer agents are particularly suitable for use in the emulsifier free polymerization as they effectively control the molecular weight without substantially affecting the emulsifier free polymerization process. Accordingly, the fluoropolymer of desired molecular weight can be obtained in a convenient and fast way and at high yield. Further, the dialkyl ether chain transfer agent can produce very pure fluoropolymers that have a low amount of extractable compounds. Additionally, the polymers so produced will generally be less susceptible to discoloration. Dialkyl ether chain transfer agents are preferably used to produce fluoropolymers that have a partially fluorinated backbone with a fluorine content of less than about 70%.
For producing fluoropolymers that have a partially fluorinated backbone with a fluorine content of higher than about 70% or that have a perfluorinated backbone, the aqueous emulsion polymerization process of the present invention preferably involves the use of lower alkanes (1 to 5 carbon atoms) such as for example methane, ethane, propane or n-pentane or hydrofluorocarbon compounds such as CH2Fxe2x80x94CF3 (R134a) to control the molecular weight of the fluoropolymer if desired.
It has been found that the emulsifier free polymerization method can be used to produce multi-modal fluoropolymers, preferably fluoropolymers with bimodal molecular weight distribution, in a single-step polymerization. Such polymers are produced preferably at a given generally constant temperature in the presence of chain transfer agents like dialkylether or lower hydrocarbon or hydrofluorocarbon with 1 to 5 carbon atoms, depending on the nature of the desired fluoropolymer.
Such multi-modal fluoropolymers may be produced by charging no or small initial amounts of chain transfer agents at the beginning of the polymerization and one or more further charges of chain transfer agents during the polymerization.
Such processes to produce multi-modal fluoropolymers are less cumbersome than producing multi-modal fluoropolymers by changing the polymerization temperature during the course of the polymerization as is known in the prior art. The multi-modal fluoropolymers typically have processing advantages and low levels of extractables.
The amount of polymer solids that can be obtained at the end of the polymerization is typically between 10% and 45% and the average particle size of the resulting fluoropolymer is typically between 200 nm and 500 nm.
Examples of fluoropolymers that are preferably produced with the process of the invention include a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of tetrafluoroethylene and vinylidene fluoride, a copolymer of tetrafluoroethylene and propylene, a copolymer of tetrafluoroethylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a copolymer of vinylidene fluoride and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a copolymer of tetrafluoroethylene, ethylene or propylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a copolymer of tetrafluoroethylene, hexafluoropropylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a copolymer of tetrafluoroethylene, vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride, tetrafluoroethylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2) and a copolymer of a copolymer of tetrafluoroethylene, ethylene or propylene, hexafluoropropylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2).
The fluoropolymers that can be produced with the process of the invention are generally amorphous or semicrystalline fluoropolymers. A fluoroelastomer is generally a fluoropolymer having elastomeric properties upon curing and will generally not display a melting peak or will have a very minor melting peak, i.e. the fluoroelastomer will generally have little or no crystallinity. Fluorothermoplasts are polymers that generally have pronounced melting peak and that generally have crystallinity. The fluorothermoplasts that can be produced according to this invention will generally be melt processible, i.e. they will typically have a melt flow index of at least 0.1 g/10 min. as measured with a support weight of 5 kg and a temperature of 265xc2x0 C. as set out in the examples below. Whether the particular fluoropolymer produced is a fluorothermoplast or fluoroelastomer, depends on the nature and amounts of the monomers from which the fluoropolymer is derived as is well known to those skilled in the art.
Fluorothermoplasts
Fluorothermoplasts that can be produced with the process of the present invention generally will have a melting point between 60xc2x0 C. and 250xc2x0 C., preferably between 60xc2x0 C. and 200xc2x0 C. and most preferably below 170xc2x0 C. Particularly desirable fluorothermoplasts that can be produced with the process of this invention include copolymers of TFE and VDF, copolymers of VDF and HFP, copolymers of TFE, E and HFP and copolymers of TFE, HFP and VDF.
Fluorothermoplasts that may be produced in connection with the present invention have the advantage of being less susceptible to discoloration, having a decreased amount of extractable compounds and having a high purity. Accordingly, the fluorothermoplasts are generally more easy to process and generally have high temperature resistance, high chemical resistance, same or improved electrical properties, good mold release and reduced amount of smell. Further, the fluorothermoplasts when extruded typically produce less die drool.
The fluorothermoplastic polymers that can be obtained with the process of the present invention can be used in any of the applications in which fluorothermoplasts are typically used. For example, the fluorothermoplasts can be used to insulate wires and cables. To produce a cable or wire insulated with a fluorothermoplast according to the invention, the fluorothermoplast can be melt extruded around a central conductor, e.g. copper wire. A conductive metallic layer may be formed around the extruded fluorothermoplast layer to produce for example a heating cable.
The fluorothermoplastic polymers produced may further be used to make hoses, in particular fuel hoses and pipes and can be used in particular in heat exchange applications. The fluorothermoplasts may also be extruded into a film or into so-called mono filaments which may they subsequently be woven into a woven fabric. Still further, the fluorothermoplasts can be used in coating applications for example to coat outdoor fabric or to make injection molded articles.
Fluoroelastomers
In addition to fluorothermoplasts, the process of the present invention also allows for making fluoroelastomers with desirable and improved properties. In particular, the fluoroelastomers produced will have a higher purity, a lesser amount of extractable compounds, will be less susceptible to discoloration, more easy to process, produce less smell. Additionally, the mechanical and physical properties of the fluoroelastomers can be maintained or improved by the process of the invention. For example, a curable fluoroelastomer produced according to the invention may have an improved compression set or improved permeation properties.
Fluoroelastomers that can be produced in connection with the present invention include perfluoroelastomers as well as elastomers that are not fully fluorinated. The fluoroelastomer may include a cure site component, in particular one or more cure sites derived from a cures site monomer (CSM) to provide a curable fluoroelastomer. Specific examples of elastomeric copolymers include copolymers having a combination of monomers as follows: VDF-HFP, VDF-TFE-HFP, VDF-TFE-HFP-CSM, VDF-TFE-PMVE-CSM, TFE-P, E-TFE-PMVE-CSM and TFE-PMVE-CSM.
To obtain a curable fluoroelastomer, a further cure site component may be included in the polymerization reaction to obtain a curable fluoroelastomer. Generally, the cure site component will be used in small amounts, typically in amounts so as to obtain a fluoroelastomer that has between 0.1 and 5 mol % of cure sites, preferably 0.2 to 3 mol % and most preferred 0.5-2 mol %.
The cure site component may comprise a nitrile group-containing cure site monomer. The cure site component can be partially or fully fluorinated. Preferred useful nitrile group-containing cure site monomers include nitrile-containing fluorinated olefins and nitrile-containing fluorinated vinyl ethers, such as depicted below:
CF2xe2x95x90CFxe2x80x94(CF2)nxe2x80x94Oxe2x80x94Rfxe2x80x94CN
CF2xe2x95x90CFO(CF2)1CN
CF2xe2x95x90CFO[CF2CF(CF3)O]g(CF2O)vCF(CF3)CN
CF2xe2x95x90CF[OCF2CF(CF3)]kO(CF2)uCN
where, in reference to the above formulas: n=1 to 5; 1=2xe2x88x9212; g=0xe2x88x924; k=1xe2x88x922; v=0 xe2x88x926; and u=1xe2x88x924, Rf is a linear or branched perfluoroalkylene or a bivalent perfluoroether group. Representative examples of such a monomer include perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene), CF2xe2x95x90CFO(CF2)5CN, and CF2xe2x95x90CFO(CF2)3OCF(CF3)CN.
Alternatively, the cure site component may comprise a fluorinated monomer having a halogen capable of participation in a peroxide cure reaction. Typically the halogen is bromine or iodine. Suitable cure-site components include terminally unsaturated monoolefins of 2 to 4 carbon atoms such as bromodifluoroethylene, bromotrifluoroethylene, iodotrifluoroethylene, and 4bromo-3,3,4,4-tetrafluorobutene-1. Examples of other suitable cure site components include CF2xe2x95x90CFOCF2CF2Br, CF2xe2x95x90CFOCF2CF2CF2Br, and CF2xe2x95x90CFOCF2CF2CF2OCF2CF2Br. Preferably, all or essentially all of these components are ethylenically unsaturated monomers.
A curable fluoroelastomer composition will generally include the curable fluoroelastomer and one or more curatives such as the peroxide and/or one or more catalysts depending on the type of cure sites contained in the curable fluoroelastomer. Suitable peroxide curatives are those which generate free radicals at curing temperatures. A dialkyl peroxide or a bis(dialkyl peroxide) which decomposes at a temperature above 50xc2x0 C. is especially preferred. In many cases it is preferred to use a di-tertiarybutyl peroxide having a tertiary carbon atom attached to peroxy oxygen. Among the most useful peroxides of this type are 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane-3 and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane. Other peroxides can be selected from such compounds as dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, xcex1,xcex1xe2x80x2-bis(t-butylperoxy-diisopropylbenzene), and di[1,3-dimethyl-3-(t-butylperoxy)-butyl]carbonate. Generally, about 1-3 parts of peroxide per 100 parts of perfluoroelastomer is used.
Another material which is usually blended with the composition as a part of the curative system is a coagent composed of a polyunsaturated compound which is capable of cooperating with the peroxide to provide a useful cure. These coagents can be added in an amount equal to 0.1 and 10 parts per hundred parts perfluoroelastomer, preferably between 2-5 parts per hundred parts fluoroelastomer. Examples of useful coagents include triallyl cyanurate; triallyl isocyanurate; tri(methylallyl isocyanurate; tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2,Nxe2x80x2-tetraalkyl tetraphthalamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinyl methyltrisiloxane; and tri(5-norbornene-2-methylene)cyanurate. Particularly useful is triallyl isocyanurate. Other useful coagents include the bis-olefins disclosed in EPA 0 661 304 A1, EPA 0 784 064 A1 and EPA 0 769 521 A1.
When the fluoroelastomer includes a nitrile containing cure site component, a catalyst comprising one or more ammonia-generating compounds may be used to cause curing. xe2x80x9cAmmonia-generating compoundsxe2x80x9d include compounds that are solid or liquid at ambient conditions but that generate ammonia under conditions of cure. Such compounds include, for example, aminophenols as disclosed in U.S. Pat. No. 5,677,389, ammonia salts (U.S. Pat. No. 5,565,512), amidoxines (U.S. Pat. No. 5,668,221), imidates, hexamethylene tetramine (urotropin), dicyan diamid, and metal-containing compounds of the formula:
Aw+(NH3)vYwxe2x88x92
where Aw+ is a metal cation such as Cu2+, Co2+, Co3+, Cu+, and Ni2+; w is equal to the valance of the metal cation; Ywxe2x88x92 is a counterion, typically a halide, sulfate, nitrate, acetate or the like; and xcexd is an integer from 1 to about 7. Still further ammonia generating compounds are disclosed in PCT 00/09603.
Fluoroelastomers, in particular VDF containing fluoroelastomers, may further be cured using a polyhydroxy curing system. In such instance, it will not be required that the fluoroelastomer includes cure site components. The polyhydroxy curing system generally comprises one or more polyhydroxy compounds and one or more organo-onium accelerators. The organo-onium compounds useful in the present invention typically contain at least one heteroatom, i.e., a non-carbon atom such as N, P, S, O, bonded to organic or inorganic moieties. One useful class of quaternary organo-onium compounds broadly comprises relatively positive and relatively negative ions wherein a phosphorus, arsenic, antimony or nitrogen generally comprises the central atom of the positive ion, and the negative ion may be an organic or inorganic anion (e.g., halide, sulfate, acetate, phosphate, phosphonate, hydroxide, alkoxide, phenoxide, bisphenoxide, etc.).
Many of the organo-onium compounds useful in this invention are described and known in the art. See, for example, U.S. Pat. No. 4,233,421 (Worm), U.S. Pat. No. 4,912,171 (Grootaert et al.), U.S. Pat. No. 5,086,123 (Guenthner et al.), and U.S. Pat. No. 5,262,490 (Kolb et al.), U.S. Pat. No. 5,929,169, all of whose descriptions are herein incorporated by reference. Another class of useful organo-onium compounds include those having one or more pendent fluorinated alkyl groups. Generally, the most useful fluorinated onium compounds are disclosed by Coggio et al. in U.S. Pat. No. 5,591,804.
The polyhydroxy compound may be used in its free or non-salt form or as the anionic portion of a chosen organo-onium accelerator. The crosslinking agent may be any of those polyhydroxy compounds known in the art to function as a crosslinking agent or co-curative for fluoroelastomers, such as those polyhydroxy compounds disclosed in U.S. Pat. No. 3,876,654 (Pattison), and U.S. Pat. No. 4,233,421 (Worm). One of the most useful polyhydroxy compounds includes aromatic polyphenols such as 4,4xe2x80x2-hexafluoroisopropylidenyl bisphenol, known more commonly as bisphenol AF. The compounds 4,4xe2x80x2-dihydroxydiphenyl sulfone (also known as Bisphenol S) and 4,4xe2x80x2-isopropylidenyl bisphenol (also known as bisphenol A) are also widely used in practice.
Prior to curing, an acid acceptor is mixed into a fluoroelastomer composition that comprises a polyhydroxy cure system. Acid acceptors can be inorganic or blends of inorganic and organic. Examples of inorganic acceptors include magnesium oxide, lead oxide, calcium oxide, calcium hydroxide, dibasic lead phosphite, zinc oxide, barium carbonate, strontium hydroxide, calcium carbonate, etc. Organic acceptors include epoxies, sodium stearate, and magnesium oxalate. The preferred acid acceptors are magnesium oxide and calcium hydroxide. The acid acceptors can be used singly or in combination, and preferably are used in amounts ranging from about 2 to 25 parts per 100 parts by weight of the fluoroelastomer.
A curable fluoroelastomer composition may comprise further additives, such as carbon black, stabilizers, plasticizers, lubricants, fillers, and processing aids typically utilized in fluoroelastomer compounding can be incorporated into the compositions, provided they have adequate stability for the intended service conditions.
Carbon black fillers are typically also employed in elastomers as a means to balance modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Suitable examples include MT blacks (medium thermal black) designated N-991, N-990, N-908, and N-907, and large particle size furnace blacks. When used, 1-70 phr of large size particle black is generally sufficient.
Fluoropolymer fillers may also be present in the curable compositions. Generally, from 1 to 50 parts per hundred fluoroelastomer of a fluoropolymer filler is used. The fluoropolymer filler can be finely divided and easily dispersed as a solid at the highest temperature utilized in fabrication and curing of the fluoroelastomer composition. By solid, it is meant that the filler material, if partially crystalline, will have a crystalline melting temperature above the processing temperature(s) of the fluoroelastomer(s). The most efficient way to incorporate fluoropolymer filler is by blending latices; this procedure including various kinds of fluoro polymer filler is described in U.S. application Ser. No. 09/495,600, filed Feb. 1, 2000.
The curable compositions can be prepared by mixing the fluoroelastomer, the curatives and/or catalysts, the selected additive or additives, and the other adjuvants, if any, in conventional rubber processing equipment. The desired amounts of compounding ingredients and other conventional adjuvants or ingredients can be added to the unvulcanized fluorocarbon gum stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as internal mixers, (e.g., Banbury mixers), roll mills, or any other convenient mixing device. For best results, the temperature of the mixture during the mixing process typically should not rise above about 120xc2x0 C. During mixing, it is preferable to distribute the components and adjuvants uniformly throughout the gum for effective cure. The mixture is then processed and shaped, for example, by extrusion (for example, in the shape of a hose or hose lining) or molding (for example, in the form of an O-ring seal). The shaped article can then be heated to cure the gum composition and form a cured elastomer article.
Pressing of the compounded mixture (i.e., press cure) usually is conducted at a temperature between about 95xc2x0 C. and about 230xc2x0 C., preferably between about 150xc2x0 C. and about 205xc2x0 C., for a period of from 1 minute to 15 hours, typically from 5 minutes to 30 minutes. A pressure of between about 700 kPa and about 20,600 kPa is usually imposed on the compounded mixture in the mold. The molds first may be coated with a release agent and prebaked. The molded vulcanizate is then usually post-cured (e.g., oven-cured) at a temperature usually between about 150xc2x0 C. and about 300xc2x0 C., typically at about 232xc2x0 C., for a period of from about 2 hours to 50 hours or more depending on the cross-sectional thickness of the article. For thick sections, the temperature during the post cure is usually raised gradually from the lower limit of the range to the desired maximum temperature. The maximum temperature used is preferably about 300xc2x0 C., and is held at this value for about 4 hours or more.
The curable fluoroelastomer compositions are useful in production of articles such as gaskets, tubing, and seals. Such articles are produced by molding a compounded formulation of the curable composition with various additives under pressure, curing the part, and then subjecting it to a post cure cycle. The curable compositions formulated without inorganic acid acceptors are particularly well suited for applications such as seals and gaskets for manufacturing semiconductor devices, and in seals for high temperature automotive uses.
The invention will now be further illustrated with reference to the following examples without the intention to limit the invention thereto. All parts and percentages are by weight unless indicated otherwise.